On Jul. 17, 1996, TWA Flight 800 exploded over the Atlantic Ocean shortly after taking off from John F. Kennedy Airport. That disaster, including its loss of the 230 persons aboard, sparked the most extensive governmental investigation in the history of aviation. As a result, it became known that the disaster started as a fuel/air explosion in the almost empty center wing tank. However, what ignited those explosive fuel/air vapors in that tank is still a mystery that may never be solved.
After theories of sabotage and accidental missile impact have been discounted, one plausible culprit remains in the form of electric sparks in the fuel system. To this effect, the possibility of a short circuit in the wiring of fuel level sensors, an electrical defect in the fuel pump that has never been found at the crash site, or a discharge of static electricity have been mentioned. Intensive investigation and tests with a duplicate aircraft have found that the temperature inside and at the central fuel tank was high because of the location of the heat producing parts of air conditioning equipment below that fuel tank.
Sources of such electrical discharges include static electricity and lightning. Both are of triboelectric origin, in that static electric charges in fuel systems are built up from such activities as refueling, while lightning typically is a discharge of electric charges from cloud to cloud and from cloud to ground and vice versa. Occasionally, lightning occurs from a clear sky, indicating the presence of electric charge concentrations in the atmosphere. Aircraft are exposed to all such phenomena and to electric induction emanating therefrom. Even if fuel tanks are inerted, fuel line systems require additional protection.
The problem is aggravated by the fact that weight considerations dictate extensive use of plastics or aluminum alloys in airborne fuel systems. Plastics include dielectrics which are prone to store electric charges.
Aluminum alloys basically have the advantage of being electric conductors. However, aluminum alloys engender other problems that become increasingly serious with the age of typical aircraft.
In particular, common practice is to alleviate buildup of electric charges in metallic fuel lines by electrically interconnecting fuel line sections in series with metallic straps around each section and electric conductors from strap to strap between adjacent sections. Such so-called "grounding systems" are typically sound when a new aircraft is delivered. However, the innate property of most metals to form thin oxide layers at their surfaces may cause such systems to deteriorate with age. The problem is particularly pronounced in the case of aluminum and aluminum alloys which form their natural oxide surface layer rapidly. Such oxide films are electrically insulating or dielectrics, as may be recalled from their high utility in electrolytic capacitors. Similarly, anodized aluminum is well known for its pleasing appearance and corrosion resistance, and represents a further example of an electrically formed electrically insulating surface film. Typical aircraft fuels are electrically insulating and therefore cannot participate in a dissipation of built up electrical charges.
In the case of conventional fuel line systems using aluminum alloys, the inevitably forming oxide surface layer eventually degrades the extremely high electric conductivity between tubing and grounding strap required for a sound prevention of electric charge buildup.
Modern aircraft typically have hundreds of locations where static charges can accumulate and many of these are attached to fuel lines. Since not all joints oxidize at the same time, joints that still are good and the metallic tubing interconnected thereby in series, can conduct considerable electric charges to a single bad joint in the series where these charges will accumulate until dielectric breakdown of sufficient intensity for sparking occurs.
Elimination of such trouble spots would go a long way in improving flight safety.